Checkbox triggered dependent form items broken for jQuery 1.7+

Problem/Motivation

In upgrading a site to use a release of jQuery newer than 1.5, the dependent checkbox code fails with a trigger value of one or true. This is because, in the ctools 1.2 release (used on the site), the dependent.js code uses the attr() function as the value of this property in:
var val = $(trigger).attr('checked') || 0;
which changed from returning true to 'checked' with the 1.6 release. This code changed in the ctools 1.3 release to:
var val = $(trigger).attr('checked') ? true : false;

Per the jQuery documentation, the property value is the preferred item to use with a checkbox element. The return value of prop() did not change with the 1.6 release.

Proposed resolution

Use the prop() function in dependent.js to determine whether the element is checked. Given the change in the 1.3 release (for returning true or false, and not 'checked' or 0 which would be the case without the change shown above), the current proposed change should have no adverse consequences on existing code. Any site using the 1.2 release of dependent.js with a 1.6+ release of jQuery would have had to modify its code to use a trigger value of 'checked' to deal with the change in return value, and would have to revert it with the next release of ctools.

Remaining tasks

Review, test, document.

User interface changes

None.

API changes

None.